In recent years, the use of digital images is advanced in the field of diagnostic imaging or medical imaging. For example, a device which radiographs or takes an X-ray image by using a semiconductor sensor has advantages as compared with a radiography system which uses conventional silver salt photography. More specifically, in such a digital radiography device, it is possible to record an image extending over a very much wider radiation exposure region, and it is also possible more easily to construct an efficient system with respect to image storing and transfer.
Moreover, by digitizing a medical image, there emerges a possibility of diagnostic form which was difficult in conventional diagnosis using silver salt photography. That is, in conventional diagnosis, in a case where plural X-ray photographs which were radiographed at different points in time during the observation of a patient's condition are compared for diagnosis, the films on which the X-ray photographs have been respectively developed are generally hung on a light box (schaukasten), and the hung films are actually compared by the diagnostician and read.
Meanwhile, in the case where the digital images are used in the diagnosis, two digital images which were radiographed at different points in time with respect to one patient are subjected to registration so that the normal anatomical structure on one digital image conforms to that of the other digital image, and then a difference process is executed on the two digital images, whereby a difference image is generated and output. Subsequently, the output difference image is compared with the pair of the two original digital images, whereby it is possible more accurately to grasp changes between the two original images.
FIG. 11 is a block diagram showing the construction of a conventional difference image generation and display device. In FIG. 11, a reference image and a comparison image which are input to a density correction unit 401 are both equivalent to medical image data of a specific region which was radiographed at different points in time. In the density correction unit 401, the image data corresponding to these images are corrected so that the distributions of the density values of the image data of these images become substantially the same. In a registration unit 402, the local relation of the anatomical structures of the specific region is acquired. Then, in a deformation unit 403, each pixel on one of these images, that is, the comparison image here, is deformed so as to overlap the corresponding pixel on the reference image, and, in a difference operation unit 404, a difference process is executed between the corresponding pixels to generate a difference image. Subsequently, the generated difference image is displayed together with the reference image and the comparison image on an image display unit 7. For example, such an operation is disclosed in Japanese Patent Application Laid-Open No. 10-155746, which corresponds to U.S. Pat. No. 5,987,345.
Here, the order of operation for the reference image and the comparison image in the above difference process is set in advance. More specifically, if it is assumed that one of these images is a past image and the other is a current image, the order of operation for these images is set to “past image”→“current image”, or “current image”→“past image”.
Typically, the difference image is used to extract a change which appears in the subject on the images which were radiographed at predetermined intervals in, for example, routine medical examinations. In such a use, as described above, a pair of images is used as the target, and it is only necessary to execute the difference process on each of these images in the predetermined order.
However, in a case of observing the progress of a specific patient, it is necessary to radiograph the target plural times during a relatively short period of time, sequentially observe the results of medical treatment, and further grasp the progress of the relevant medical treatment by tracing the relevant features in the sequence of images. In such conventional progress observation, there is no method of effectively generating a diachronic difference image.